The present invention relates to a pneumatic tire, more particularly to a tread reinforcing structure being capable of improving tire noise and rolling resistance.
In order to improve high-speed durability of a pneumatic tire especially a radial tire for passenger cars, as shown in FIG. 7A, a single-layered full-width band (b1) over a breaker (a) is widely used. Here, a band means a cord layer whose cord angle is less than about 10 degrees usually less than 5 degrees with respect to the tire equator, and a breaker means a cord layer whose cord angle is more than 10 degrees usually more than 15 degrees.
Such a single-layered full-width band (b1) can reduce a road noise around a frequency of 250 Hz heard on the inside of a vehicle during running. By using a higher tensile modulus cord in the band (b1), such a road noise may be reduced more, but a pass-by noise heard on the outside of a vehicle during running tends to increase. It is effectual in reducing the pass-by noise to use axially spaced edge bands (b2) on the edge portions of a breaker (a) as shown in FIG. 7B. Such axially spaced edge bands (b2) however, tend to increase the rolling resistance of the tire.
It is therefore, an object of the present invention to provide a pneumatic tire, in which the rolling resistance and tire noise such as road noise and pass-by noise are improved.
According to the present invention, a pneumatic tire comprises
a carcass extending between bead portions through a tread portion and sidewall portions,
a breaker disposed radially outside a crown portion of the carcass,
a pair of axially spaced high modulus edge bands disposed radially outward of edge portions of the breaker and each made of one or more high modulus cords having a sectional area SH and a 2% modulus MH and wound substantially parallel to the tire circumferential direction at a cord count DH,
a low modulus middle band disposed radial outward of the breaker between the high modulus edge bands and made of one or more low modulus cords having a sectional area SL and a 2% modulus ML less than the 2% modulus MH and wound substantially parallel to the tire circumferential direction at a cord count DL, and
an elongation resistance index PH of each of the high modulus edge bands, which is defined as the product of the 2% modulus MH in N/sq.mm, the sectional area SH in sq.mm and the cord count DH per 1 cm width divided by 100, being in a range of 160 to 700.
Further, an elongation resistance index PL of the low modulus middle band, which is defined as the product of the 2% modulus ML in N/sq.mm, the sectional area SL in sq.mm and the cord count DL per 1 cm width divided by 100, may be set in a range of not less than 30 but less than 160.